Semiconductor production equipment, liquid crystal production equipment, electronic computers, printed substrate production equipment, and so forth generally carry out production in a clean environment such as a clean room. Greases with a reduced oil scattering behavior at the lubricated part, i.e., with a reduced amount of dust emission, are preferably used as the grease filled as the lubricant in the motion guiding devices (for example, table devices that use linear guides, ball screws, rolling bearings, or cross roller rings) that are incorporated in devices used in such environments.
As the precision equipment workpieces, e.g., glass substrates, transported by clean robots have grown in size, the clean robots that transport such precision equipment workpieces have also grown in size. In addition to the low dust emission behavior mentioned above, a high load bearing capacity is therefore also required of the grease for lubricating a motion guiding device incorporated in a large-scale clean robot. A large amount of so-called flaking is produced in the Z-axis of the clean robot and on components in indirect regions when a grease with a low load bearing capacity is used to lubricate a motion guiding device incorporated in a large-scale clean robot.
Fluorinated greases are already known to be greases that exhibit an excellent performance in terms of a low dust emission behavior. Greases that use a perfluoropolyether (PFPE) as the base oil and a polytetrafluoroethylene (PTFE) with a prescribed particle diameter as the thickener are generally commercially available. Greases that use a PFPE base oil certainly exhibit an excellent dust emission behavior, but the PFPE used as the base oil is expensive and high production costs have thus been a problem when PFPE is used for grease production. The load bearing capacity of these fluorinated greases has also been a problem.
The lubricating grease provided in Patent Document 1 is a grease that has an excellent performance with regard to low dust emission; this lubricating grease incorporates a mineral oil or a poly-α-olefin as its base oil and incorporates lithium stearate and lithium 12-hydroxystearate as thickeners. A grease is provided in Patent Document 2 that incorporates a synthetic hydrocarbon oil or an ether oil as its base oil and a urea compound as a thickener and that has a metal element content and worked penetration in prescribed ranges. A grease composition is provided in Patent Document 3 that uses an ester-type base oil and a poly-α-olefin as its base oil and an aliphatic diurea as a thickener.
However, the load bearing capacity has also been a problem for these low dust-emission greases.
The art of incorporating an extreme-pressure agent as an additive for the purpose of raising the load bearing capacity of grease compositions is generally known. However, the dust emission behavior may deteriorate when an extreme-pressure agent is added to a grease composition, and it has been quite difficult to prepare a grease composition that satisfies both the demand for a low dust emission behavior and the demand on the load bearing capacity.
Against this background, research and development has also been carried out into grease compositions that would be endowed with an excellent load bearing capacity and an excellent performance with regard to low dust emission. A grease composition is provided in Patent Document 4 that takes the load bearing capacity into account in addition to a low dust emission behavior; this grease composition uses a poly-α-olefin having a kinematic viscosity at 40° C. of 60 to 320 mm2/s for its base oil and uses a lithium salt of a hydroxyl group-free C10-22 fatty acid as a thickener. However, there is still room for additional improvement in the load bearing capacity even with this grease composition.
Patent Document 1: Japanese Patent Application Laid-open No. H6-330070
Patent Document 2: Japanese Patent Application Laid-open No. H11-166191
Patent Document 3: Japanese Patent Application Laid-open No. 2005-272764
Patent Document 4: Japanese Patent Application Laid-open No. 2009-256401